Fossil Focus: Tyrannosaurs

Introduction:

Thanks to Tyrannosaurus rex, the tyrannosaurs are among the most famous of the extinct dinosaur groups. They receive a disproportionate amount of attention in the media and hold a firm place in the public imagination. However, this also means that more misconceptions and out-of-date ideas are promoted for this group than any other, and the excess of attention detracts from the fact that they are a genuinely interesting clade of animals. In fact, thanks to a great deal of research effort, we may know more about tyrannosaurs than any other group of dinosaurs from the Mesozoic era (252 million to 66 million years ago). This alone makes them a key part of palaeontology.

All tyrannosaurs were carnivores, and although the most famous forms from the last part of the Cretaceous period (146 million to 66 million years ago) were huge (12 metres or more in total length) with enlarged heads and small arms, early forms were much smaller (3 metres or so) and were more generic in appearance (Fig. 1). Tyrannosaurus rex is the largest known tyrannosaur and lived at the very end of the Cretaceous in what is now North America.

Figure 1 — A stunning and complete specimen of a subadult Gorgosaurus on display in the Royal Tyrrell Museum of Palaeontology in Drumheller, Canada.

Relationships and classification:

Here the informal term ‘tyrannosaur’ is being used here to cover all members of the Tyrannosauroidea. This is includes all things more closely related to Tyrannosaurus than to other theropods (the primarily carnivorous group of bipedal dinosaurs that includes modern birds). The tyrannosaurs make up one of the early branches of the great theropod group called the Coelurosauria, and some of the nearest relatives of the tyrannosaurs are the mostly tiny compsognathids and the ostrich-like (and actually mainly herbivorous) ornithomimosaurs.

About 30 species of tyrannosaurs have been described to date (though some are controversial), and new species are currently being described at a very rapid rate — over two per year since 2010.

Two major controversies of tyrannosaur classification actually centre around Tyrannosaurus rex itself. Some researchers — although not many — suggest that the Asian Tarbosaurus bataar should be considered a second species of its North American cousin and so should be listed as Tyrannosaurus bataar. Similarly, a number of researchers suggest that a dwarf tyrannosaur termed Nanotyrannus lived alongside Tyrannosaurus, but most are now confident that specimens attributed to this genus are in fact young examples of Tyrannosaurus.

Some of the earliest tyrannosauroids belong in a branch of the group called the Proceratosauria, a set of small tyrannosaurs from the Jurassic Period (200 million to 145 million years ago) that had large bony crests on their skulls (Fig. 2). Later tyrannosaurs are part of the Tyrannosauridae, and are generally larger and have bigger skulls. The most recognizable forms are the late Cretaceous groups the Albertosaurinae and Tyrannosaurinae. Members of both are large-bodied (including multi-tonne) animals with still larger heads and greatly reduced arms. The tyrannosaurines are exceptional examples of this overall trend, and they include Tyrannosaurus.

Figure 2 — The crested skull of the proceratosaurid Guanlong from the Middle Jurassic of China. Image by Dave Hone, used with permission from Xu Xing, Chinese Academy of Sciences.

Anatomy:

All tyrannosaurs had fused nasal bones in their skulls that provided support, and this feature helps to characterize the group. Similarly, all tyrannosaurs had two kinds of teeth in their jaws: those down the sides and in the lower jaw were much like those of other carnivorous theropods (if often rather stronger), but those in the very front of the skull (in the upper jaw) were near circular in cross-section and packed tightly together.

The tyrannosaurids, and especially the tyrannosaurines, had proportionally enlarged heads that were more robust than those of other theropods and capable of dealing with the forces of strong bites. Proceratosaurs had enlarged crests on the midline of their skulls (Fig. 2), and most other tyrannosaurids had some kind of miniature horns over the eyes (Fig. 3).

Figure 3 — A skull of Gorgosaurus showing the small spikes over the eyes and gnarled texture to the nasal bones, indicating that some form of boss or keratinous (horny) mass probably lay on the skull in life.

As the heads of the later tyrannosaurs got larger, the arms got smaller and the number of fingers reduced from three to two. These arms were still relatively strong, but would have had little use and do not seem to have been integral to predation.

The hips of later tyrannosaurs show a characteristic pattern. The ilium (the bone of the pelvis that is anchored to the spine) becomes ‘pinched’ so that when seen from above, the two sides nearly meet in the middle. Second, the pubis (the long bone that projects down and forwards from the pelvis) has an especially expanded end, called a pubic boot.

Tyrannosaurs had proportionally long upper legs (femurs) and feet (metatarsals) compared to most other dinosaurs. Furthermore, as in many other excellent theropod runners, the middle metatarsal of the foot was constricted near the ankle, giving the foot a characteristic shape. This is called the arctometatarsal (Fig. 4) and is linked to reduced movement between the bones of the foot when moving quickly, which makes movement more efficient.

Figure 4 — The specialized arctometatarsus in the foot of Tyrannosaurus. Note how the middle bone is almost triangular.

Tyrannosaurs had feathers. Both the small Dilong and the large (more than 7 metres in total length) Yutyrannus from China were preserved with feathers. These were simple filaments, and for Yutyrannus at least, they covered the whole animal. It is likely that all tyrannosaurs had at least some feathers on their bodies, and this includes even the largest tyrannosaurines.

Lifestyle:

Much has been written about the lifestyle and behaviour of various tyrannosaurs but much of it rather fanciful, overstated or out of date. All tyrannosaurs were certainly carnivores, and moreover they were both predators and scavengers. There is excellent evidence for both behaviours and we would expect both to have occurred anyway as it is normal for large carnivorous animals. Several hadrosaurs (duck-billed dinosaurs) have been found with healed injuries following bites from tyrannosaurs (one even has a tooth embedded in its tail), and one hadrosaur carcass shows signs that a tyrannosaur had fed on it after it was already partially buried. This must have happened long after the hadrosaur died and so indicates scavenging (Fig. 5).

Figure 5 — Bite marks from a large tyrannosaur on the humerus of a buried hadrosaur from Mongolia, indicating scavenging. Numerous sub-parallel bites are present on the bone where the specialised teeth in the front of the upper jaw have been dragged across the surface.

Several tyrannosaur skeletons show evidence of having been fed on by other tyrannosaurs. Tyrannosaurus was cannibalistic, but probably more as a result of taking opportunities to scavenge than as a matter of course. They were unlikely to have normally hunted each other – other large carnivores are dangerous targets.

Tyrannosaurs had generally good vision and sense of smell, which would have assisted in finding food. Later tyrannosaurs had exceptionally powerful and even literally bone-crunching bites, and these would have been used to dispatch prey. Despite popular illustrations showing tyrannosaurs targeting huge adult dinosaurs, preserved stomach contents, coprolites (fossilized dung; Fig. 6.) and healed marks on other dinosaurs suggest that they preferred younger animals. Even the giants had relatively long legs and were efficient, if not necessarily fast, runners. Earlier tyrannosaurs and young tyrannosaurids would have been faster, but lacked the powerful bite of adults and may have targeted proportionally smaller prey.

Figure 6 — Bone fragments of a young hadrosaur in a coprolite (fossilized dung) attributed to Tyrannosaurus. Image by Karen Chin, used with kind permission of Royal Saskatchewan Museum.

Tyrannosaurs would have fed on prey or scavenged carcasses using a number of techniques. Their heavy bites would have separated joints and broken up smaller prey but when feeding on larger items, a strong bite could take off chunks of bone. However tyrannosaurs would mostly have dragged their specialised front teeth across the bones to remove meat (Fig. 5).

Fossils of numerous tyrannosaurs with healed bites on their faces suggest that they regularly fought one another, perhaps over territory, mates or food. Multiple animals found together hint that some animals in some species may have lived in groups on occasion, but this is little studied at the moment.

Fossil Record:

The first tyrannosaurs appeared in the Middle Jurassic period around 166 million years ago, with the last species coming at the very end of the Mesozoic, around 66 million years ago. The group went extinct, with the rest of the non-avian dinosaurs, as part of the great mass extinction at the end of the Cretaceous.

Tyrannosaur fossils are mainly known from Europe, East Asia and North America. Controversial specimens that may or may not represent tyrannosaurs also occur in Australia and South America, and it is possible that the animals had a near-global distribution. By the Late Cretaceous they were apparently restricted to the northern hemisphere, with specimens and species being especially prevalent in Mongolia and Alberta, Canada. An increasing amount of material has also come from Utah, U.S.A. and China in recent years.

Although a number of tyrannosaurs are known from only very fragmentary remains (including Proceratosaurus, Zhuchengtyrannus and Nanuqsaurus), others are known from numerous and exceptional specimens such as Albertosaurus which includes over 20 specimens found together in a single quarry. The Chinese Yutyrannus is known from three almost complete skeletons, two of which are preserved together and all of which preserve feathers. The Canadian Gorgosaurus is known from a number of exceptionally preserved specimens, including one that is apparently totally complete to the last bone in the skeleton (Fig. 1) — a real rarity in dinosaurs. Collectively there are tyrannosaurs showing injuries, some with stomach contents, we can see variation within the species, and a wide range of sizes – from young juveniles through to adults.

Summary:

In part thanks to their notoriety, the tyrannosaurs — and Tyrannosaurus in particular (Fig. 7) — have enjoyed a huge amount of research, making them one of the best studied and understood groups of non-avian dinosaurs. However, despite being the main frame of reference for the media when it comes to dinosaurs, they are not just model organisms, but interesting and unusual animals. No other terrestrial predator has evolved such a proportionally large skull and strong bite, and they were among the largest terrestrial carnivores of all time.

Figure 7 — A mounted cast of Tyrannosaurus in a hypothesized sitting posture on display in Tokyo.